Earticle

The applicability of the two types of magnetic separation system, high gradient magnetic separation (HGMS) and Magneto Archimedes method were studied considering the magnetic susceptibility of targeted substances. It was noted that the combination of the two methods can control the almost all the substances, paramagnetic and diamagnetic in addition to ferromagnetic substances. The principle of the methods was given and the conceput of the magnet force control technology is discussed. The practical applications of the technique were introduced together with the new application of HGMS.

In this review article, we focus on various co-evaporation technologies developed for the fabrication of high performance REBa2Cu3O7-δ (RE: Y and Rare earth elements, REBCO) superconducting films. Compared with other manufacturing technologies for REBCO films such as sputtering, pulsed laser deposition (PLD), metal-organic deposition (MOD), and metal organic chemical vapor deposition (MOCVD), the co-evaporation method has a strong advantage of higher deposition rate because metal sources can be used as precursor materials. After the first attempt to produce REBCO films by the co-evaporation method in 1987, various co-evaporation technologies for high performance REBCO films have been developed during last several decades. The key points of each co-evaporation technology are reviewed in this article, which enables us to have a good insight into a new high throughput process, called as a Reactive Co-Evaporation by Deposition and Reaction (RCE-DR).

MgB2 superconductor is highly sensitive to the Mg content. Even if the samples are synthesized with the appropriate looking stoichiometric ratio, the heat treatment leads to the loss of Mg either to ambiance or to MgO. To avoid it, either excess Mg is added in the starting powder or sealed ampoule annealing is employed. In this paper the effect of open Mg sintering ambiance on the ex-situ MgB2 was studied to enhance its superconducting properties. The open Mg ambiance was created to avoid any overpressure of Mg by providing a hole in Fe tube used as sample holder. The decrease in resistivity of the synthesized sample was observed through the increased temperature dependence of electron-phonon interactions. A clear enhancement in the superconducting cross-sectional area and hence the in-field critical current density is obtained

In power cables as one of the important power applications adopting HTS tapes, a good insulation should be kept at its optimum performance. As an insulation material for superconducting device applications, polypropylene laminated paper (PPLP) is now widely used instead of the conventional Kraft paper. In addition to its dielectric property, the insulation material should also possess superior mechanical property at cryogenic temperatures and operability that is necessary for the insulation winding process. This study aims to evaluate the mechanical property of the PPLP material at ambient and cryogenic temperatures. At cryogenic temperature, the failure stress of PPLP increased significantly as compared with that at ambient temperature. The failure stress at both temperatures depended upon the sample orientation to the load application.

Superconducting equipment has been actively investigated for securing the environment and energy technology (ET) in various parts of the world. Despite these movements, a high voltage cryogenic bushing, which plays an important role of interconnection between the electric power systems and superconducting devices, has not been fully developed due to severe insulation requirements. A gas insulated cryogenic bushing has been investigated as one of our projects since 2010. As a basic step to obtain the design parameters for cryogenic bushing, we focused on the surface flashover characteristics of glass fiber reinforced plastic (FRP) in several insulation gases. For the surface flashover tests, several insulation gases including SF6, CF4 and N2 gas were prepared. Various length of FRP specimens were fabricated in order to obtain the fundamental data for creepage distance of FRP. The first specimen group was from 2 mm to 10 mm with 2 mm intervals and the second specimen group was from 20 mm to 100 mm with 20 mm intervals. And the gas pressure was varied from 1 bar to 4 bar. An AC overvoltage test and a lightning impulse test were performed. Then the experimental results of surface flashover were obtained and analyzed. Based on these results, it would be possible to design the optimum creepage distance of FRP in a cryogenic bushing.

The measurement of the coefficient of thermal expansion (CTE) of polypropylene laminated paper (PPLP) as electric insulating material is important for its practical superconducting device application. The thermal strain induced to HTS tapes and its insulating material during cooling from room temperature might largely affect the critical current (Ic) of HTS tapes. In this study, the thermal contraction of PPLP material was measured during cooling from 300 K to 77 K using double extensometers. Initially, the CTE of a brass tape was measured and it was compared with a reference data. It was found that the measured thermal expansion data of the brass material approaches that of the reference one. Based on the results, it was then confirmed that the measurement technique could be applied to thin and flexible samples. Therefore, the same measurement procedure was applied to PPLP material using double extensometers. As a result, the linear CTE of the PPLP at 77 K has been measured to be ~ 15.3 x 10-6/K. Also, it was found that the thermal contraction characteristics of PPLP was dominated by polypropylene on the cross direction (higher thermal contraction) while it was dominated by Kraft paper on the machine direction (lower thermal contraction). Overall, this measurement procedure could be adopted for the determination of CTE of flexible materials such as PPLP.

We report the solid-liquid phase equilibria on the GdBa2Cu3O7-δ (GdBCO) stability phase diagram in low oxygen pressures (PO2) ranging from 1 to 100 mTorr. On the basis of the GdBCO stability phase diagram experimentally determined in low oxygen pressures, the isothermal sections of three different phase fields on log PO2 vs. 1/T diagram were schematically constructed within the Gd2O3-Ba2CuOy-Cu2O ternary system, and the solid-liquid phase equilibria in each phase field were described. The invariant points on the phase boundaries include the following three reactions; a pseudobinary peritectic reaction of GdBCO ↔ Gd2O3 + liquid (L1), a ternary peritectic reaction of GdBCO ↔ Gd2O3 + GdBa6Cu3Oy + liquid (L2), and a monotectic reaction of L1 ↔ GdBa6Cu3Oy +L2. A conspicuous feature of the solid-liquid phase equilibria in low PO2 regime (1 - 100 mTorr) is that the GdBCO phase is decomposed into Gd2O3 + L1 or Gd2O3 + GdBa6Cu3Oy + L2 rather than Gd2BaCuO5 + L well-known in high PO2 like air.

A high-Tc superconducting (HTS) DC application has advantages such as the ultimately lower loss, more compact dimensions, and large capacity compared to AC application. In order to optimize the insulation design of a HTS DC machines, it is important to understand the high voltage insulation and materials at cryogenic temperature. Polypropylene laminated paper (PPLP) has been widely used as an insulating material for HTS AC machines. However, the fundamental data under DC voltage have not been revealed satisfactorily until now. In this paper, we will discuss mainly on the breakdown and dielectric characteristics ofPPLP in liquid nitrogen (LN2). The polarity effects of DC and impulse voltage were studied by using the semi-rod to cylindrical electrode. The volume resistivity of PPLP in LN2 was studied. Also, the space charge distribution at room temperature was studied. However, it is necessary to study this topic at cryogenic temperature in the near future.

This paper presents an experimental and numerical study on current limiting characteristics of a fault current controller (FCC). The FCC consists of an AC/DC power converter, a superconducting coil, and a control unit. Even though some previous researches proved that the FCC could adjust the fault current level, the current limiting characteristics by the superconducting coil should be investigated for design of the coil. In this paper, four kinds of model coils were tested; 1) air core, 2) iron core without any bias, 3) reversely magnetized core (RMC) using permanent magnets, and 4) RMC using an electromagnet. Based on a comparative study, it is confirmed that a RMC by an electromagnet (EM) could increase the effective inductance of the coil. In this paper, a numerical code to simulate the HTS coil with RMC was developed. This code can be applied to design the HTS coil with active reversely magnetized bias coil.

An HTS (high temperature superconducting) coil without insulation has been investigated since a metallic insulation was suggested in the mid-1980s. The advantage of an insulationless HTS pancake coil is that it is more stable than an insulated HTS pancake coil. This paper focuses on the various characteristics of the insulationless HTS pancake coil related with stability, especially under the external magnetic field. Because HTS pancake coil may be influenced by the external magnetic field in a real operational environment of electrical devices. First, charge-discharge test was performed for the characteristics evaluation of the insulationless HTS pancake coil as compared with insulated HTS pancake coil in liquid nitrogen at 77 K. Based on the experiment results, characteristics evaluation of the insulationless HTS pancake coil was implemented under the external magnetic field. In order to carry out the experiment, we have fabricated a cylindrical solenoid coil to apply the magnetic field. The various characteristics of the insulationless HTS pancake coil were evaluated for charge-discharge and over-current conditions. This paper proves that the insulationless HTS pancake coil is more stable than the insulated HTS pancake under the external magnetic field.

The selection of the temperature sensor in the cryogenic system depends on the temperature range, shape and accuracy. The accuracy of the temperature sensor is essential to improve the reliability of experiment. We have developed the variable temperature cryostat using a two-stage cryocooler. In order to reduce heat load, thermal shield is installed at the first stage with MLI (Multiple layer insulation). We have also developed the sensor holder calibrating more than twenty sensors at the same time for saving time and money. The system can calibrate sensor at variable temperature by controlling electric heater. In this paper, we present design and fabrication of variable temperature cryostat and representative result of Cernox sensor calibration.

This paper describes experimental study on GM-type pulse tube refrigerator (PTR). In a PTR, the pulse tube is only filled with working gas and there exists secondary flow due to a large temperature difference between cold-end and warm-end. The stability of secondary flow is affected by orientation of cold-head and thus, the cooling performance is deteriorated by gas mixing due to secondary flow. In this study, a single stage GM-type pulse tube refrigerator is fabricated and tested. The cooing performance of the fabricated PTR is measured as varying cold-head orientation angle and the results are used as reference data. Then, we divided interior space of pulse tube into three segments, and fixed the various size of screen mesh at interface of each segment to suppress the performance degradation due to secondary flow. For various configuration of pulse tube, no-load test and heat load test are carried out with the fixed experimental condition of charging pressure, operating frequency and orifice valve turns. From experimental results, the fine screen mesh shows the effective suppression of performance degradation for the large orientation angle, but the use of screen mesh cause the loss of cooling capacity rather than the case of no insertion into pulse tube. It should be compromised whether the use of screen mesh in consideration of the installation limitation of a GM-type pulse tube refrigerator